The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx‐2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo‐nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth‐integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

No abstract

Four large, open-ocean diatoms from the Southern Ocean (Actinocyclus sp., Thalassiosira sp., Fragilariopsis kerguelensis, and Corethron pennatum) were grown in natural (low iron) Southern ocean seawater with increasing Fe concentrations. With increasing dissolved iron (Fe diss ) concentrations, the growth rates increased three-to sixfold. The species with the smallest cells had the highest growth rates. The half-saturation constants (K m ) for growth were low (0.19-1.14 nmol L Ϫ1 Fe diss ), and close to the ambient Fe diss concentrations of 0.2 nmol L Ϫ1 . The range in K m with respect to Fe diss also varied with the size of the diatoms: the smallest species had the lowest K m and the largest species had the highest K m . As Fe diss concentrations decreased, silicate consumption per cell increased, but nitrate consumption per cell decreased. Phosphate consumption per cell varied without clear relation to the dissolved iron concentrations. The differences in nutrient consumption per cell resulted in marked differences in elemental depletion ratios in relation to Fe diss concentrations, with the depletion ratios being most affected by iron limitation in the largest cells. These experimental findings are in agreement with previous laboratory and field studies, showing the relatively high requirements of large diatoms for Fe. The size-dependent response of the diatoms with respect to nutrient depletion is a good illustration of the effects of Fe on silicate, nitrate, and phosphate metabolism.

Different stages of the automortality in phytoplankton have been studied applying flow cytometry. These stages are, in order of expression : (1) compromised cell membranes, (2) degradation of the photosynthetic pigments and reduction of the photosynthetic activity, (3) fragmentation of the genomic DNA. The integrity test of the cell membranes is based on the inability of the DNA-specific stain SYTOX Green to pass into cells with intact plasma membranes. The reduction in photosynthetic activity was examined by sorting "%C-labelled phytoplankton cells differing in viability. Finally, DNA fragmentation was traced by measuring changes in genomic DNA. The different phytoplankton species tested showed a great variety in response when grown under the same conditions, but there was also considerable intraspecific variation. Unstained cells, fully stained cells (equivalent to full staining of genomic DNA in fixed cells) and cells with intermediate fluorescence signal occurred together within the same culture. The photosynthetic activity in cells with a reduced viability dropped by as much as 60 % relative to that of the viable cells. In the subsequent stage, when photosynthetic pigments were fully degraded, this value dropped further to around 10 %. Cells in this stage also showed subdiploidy as a result of genome fragmentation. Field tests using samples of phytoplankton collected in the North Atlantic Ocean (40m N, 23m W) during spring showed staining properties similar to those found in cultures grown at suboptimal growth conditions. The percentage of non-viable cells varied considerably (ranging from 5 % to 60 %) between the various phytoplankton groups present. The lowest value was observed for Synechococcus, but some pico-eukaryotes showed percentages as high as 60 %. Moreover, the viability varied with depth (light level) and over a light-dark cycle. The present findings suggest the existence of a (genetically based) uniform process of automortality in phytoplankton. Non-viable cells are a substantial component of the oceanic phytoplankton, affecting the food-web structure and species succession.